1. Field of the Invention
The present invention relates to binary digital image processing methods and more particularly to improved methods for enlarging binary digital images.
2. Description of the Prior Art
The following are systems representative of the prior art.
K. L. Anderson, F. C. Mintzer, and J. L. Mitchell, in U.S. Pat. No. 4,569,081 "Fast algorithm for enlarging an image by 1/5 in both dimensions,", assigned to a common assignee, teach a method for expanding a binary image including the steps of: storing the image in bit sequence; inserting for each string of 5 bits along a first axis one or more expansion bits, to convert each said string of 5 bits to a string of 6 bits; assigning a value to each expansion bit generated by the above step; inserting one or more rows of expansion bits for each 5 rows of bits along a second axis of the image to convert each 5 rows of bits to 6 rows of bits along the second axis of the image; assigning a value to each expansion bit generated by the above steps; storing the enlarged image generated by the above steps.
This prior art system is a precursor to the present invention. Although the patent does deal with expansion of an image by a factor of 1.2 which is between 1.0 and 2.0, the patent does not show an interactive system in which an operator may select an expansion factor and which includes the steps of rotating the image, inserting blank rows in the image to convert i rows of bits to j rows of bits where i&lt;j&lt;=2i and assigning a value to each bit in each inserted row.
U.S. Pat. No. 4,303,948 describes an image enlargement process. The enlargement procedure differs from the method of the present invention in that it is a two-step process: the image is first expanded by an integer factor and then reduced by a fractional factor. Expansion is accomplished by merely replicating bits, whereas the present invention interpolates to obtain the bit values placed in an inserted row. Since the reduction algorithm of the patent does not accomodate an arbitrary reduction factor, the expansion algorithm also only handles a rather limited set of expansion factors. In the method of the present invention, any expansion factor can be closely approximated, although in some cases it may be necessary to apply the algorithm more than once. The patented expansion algorithm applies the same expansion factor in both the horizontal and vertical dimensions. The method according to the present invention expands in one dimension. To expand in both dimensions it is necessary to rotate the image 90 degrees, expand vertically (to get the horizontal expansion), rotate the image back to its original orientation, and then expand vertically again. Thus it is trivial to have different expansion factors for the horizontal and vertical axes. The patented algorithm requires two page memories; it apparently does not allow expansion in situ.
U.S. Pat. No. 4,254,409 describes an image enlargement process designed to do page composition using alphanumerics and simple graphics, rather than to operate on an already-composed image containing arbitrary data. It assumes that objects in the image are described as a series of graphics elements, each of which has a corresponding precanned procedure for enlarging it. It thus assumes some knowledge about what the image represents.
U.S. Pat. No. 4,409,591 is similar to U.S. Pat. No. 4,254,409. It operates only on a specified set of coded symbols (basically alphanumerics) rather than on arbitrary image data. Like U.S. Pat. No. 4,254,409, it assumes that the characters are described as a series of graphics elements; in this case enlargement is done by having precanned dot patterns available to create each element at any of a finite number of larger sizes.
U.S. Pat. No. 4,367,533 describes enlargement of images by a process which appears to be replication of pixels.
U.S. Pat. No. 4,357,604 describes a hardware method for enlarging the dot patterns corresponding to coded characters (not image data, although the image could be represented as coded data using a programmable symbol set) prior to display. The enlargement factor appears to be restricted to integer values. Enlargement is by replicating pels in one dimension and by leaving extra space between pel columns in the other dimension.
U.S. Pat. No. 4,267,573 operates by transforming images (e.g. to a log spiral coordinate system). This is much more complex than the method of the present invention.
U.S. Pat. No. 4,153,896 scales the image first in one dimension and then in the other. This patent relies on hardware, that it describes, to read an image in either scan dimension, so that the rotations do not actually have to be performed. It is not appropriate for direct implementation in software, since most computers do not have this hardware capability. It is capable of scaling (enlarging or reducing) by an arbitrary factor. The enlargement algorithm is equivalent to replicating pels.
Although the prior art discussed above relates generally to the field of the present invention, none of the art teaches nor suggests the method of the present invention.